I think i'm gonna go with one of the rectangular 38" cases and paste some music company stickers onto it. Might be more of a target to get ripped off, but it will mostly be out of sight in the trunk. They look enough like a guitar bag that I could probably walk around anywhere without anybody looking past the Fender Guitar, Blue Steel strings and the like. I even play guitar, and have a couple at home. And it is still a purpose built rifle case, I don't think a few well placed stickers could make Wicsonsin law deem it not an "appropriate" case for a firearm.
I'll think about it some more, and eventually post pictures of what I finally come up with.